1. Field of the Invention
The present invention relates to a conductor for use in a thin flat cable used as an internal wiring in various kinds of electronic application products such as an electronic desk-top computer, a printer, a copying machine and the like, and a method and equipment for manufacturing the conductor.
2. Description of the Related Art
Conventionally, to manufacture a thin flat cable, for example, as shown in FIG. 1, a plurality of flat conductors 11 are arranged at a predetermined distance from one another and in parallel to one another, and two polyester films 21 each having polyester system adhesives attached thereto are laminated onto the respective flat conductors from above and below with the adhesive attached surfaces of the two films facing each other, thereby forming an insulation layer 2.
This type of laminated flat cable provides a small space factor and thus is advantageous in reducing the size of the electronic application products and, therefore, it is used in various electronic application products. Especially, as shown in FIG. 2, a method of use of such a laminated flat cable, in which the conductor 1 on one surface of the end portion of the flat conductor 11 is exposed and the flat conductor 11 is then used in combination with a removable connector (which allows insertion and removal), has been used more and more frequently, because this method is advantageous in handling, that is, this method allows easy change and replacement of parts, wirings and the like.
As the method using a combination of the removable connector with the laminated flat cable of this type has become more popular, there has arisen a requirement for stable contact reliability with respect to the removable connector which needs to be frequently inserted into and removed from the laminated flat cable of this type for a long period of use. In addition, as the results of various tests for enhancement of the contact reliability with respect to the removable connector, it has been found that the contact reliability for a long period of use is closely related with a tin plating or a tin alloy plating such as a tin/lead alloy plating treated on the conductor.
In other words, as a conductor to be used in the laminated flat cable of this type, when a tin plated or tin alloy plated copper foil is slit into a predetermined width and is then used, that is, such slit copper foil is used in a state in which the slit surface thereof does not include any plating thereon, then the conductor of this type raises a problem as to the contact reliability for a long period of use. On the other hand, when the conductor is used after the slit surface thereof is also tin plated or tin alloy plated, then no problem as to the contact reliability arises even when the conductor is used for a long period of time. At the same time, however, if excessive tin plating or tin alloy plating is applied to the slit surface, then such excessive tin plating or tin alloy plating can contact a guide member or the like of a laminator and thus can be scraped off to thereby facilitate the production of tin plating or tin alloy plating refuse when the flat cable is manufactured. That is, the excessive tin plating or tin alloy plating gives rise to another problem.
As described above, if a conductor including a portion with a tin plating or a tin alloy plating having a thickness of less than a predetermined value is used as a conductor, then there arises a problem regarding to the contact reliability when the conductor is used for a long period of use. Also, if a conductor including a portion with a tin plating or a tin alloy plating having an excessively great thickness is used, then there arises another problem that plating refuse is easy to produce. Therefore, not only to provide a good contact reliability for long use but also to prevent production of the plating refuse, it is necessary to use, as a flat conductor for use in a flat cable, a conductor which is substantially uniformly tin plated or tin alloy plated with a predetermined thickness over the whole periphery thereof.
FIG. 4 is a block diagram of a conventional method of manufacturing a tin plated or tin alloy plated flat conductor. For example, a tin plated mother material of a round wire having a diameter of 0.9 mm is supplied to an elongating machine. In the elongating machine, the mother material is elongated until it reaches a section area almost identical with the size (width and thickness) of a desired flat conductor to be obtained after rolling. Next, it is supplied to a rolling machine, in which it is rolled to a predetermined flat shape. Next, the thus obtained flat material is softened by use of a softening device to thereby produce a tin plated flat annealed copper wire which is wound by a winding device.
However, the plated copper wire after elongation is turned into a hard-drawn copper wire by the elongating operation. That is, in the conventional manufacturing method of the above type, the plated hard-drawn copper wire, as it is elongated, is rolled. For this reason, the plating and the hard-drawn copper wire disposed inside the plating differ too much in elongation rate when the plated hard-drawn copper wire is rolled and thus, as shown in FIG. 5, in the straight angle section of the wire after it is rolled, the plating thicknesses of the two side surfaces of the wire are greater than the flat surface of the wire, and burrs can be easily produced in the edge portion A thereof. This section shape is maintained also by a softening operation to be performed after the rolling operation and thus, when a flat cable is manufactured, such thicker plated portions and burrs can get into contact with a guide member or the like and can be scraped off thereby, which facilitates the production of metal refuse. The thus produced metal refuse gives rise to short circuits between the conductors of a flat cable. Also, this can often raise a problem with respect to the contact reliability of the flat cable between a removable connector for a long period of use.